1. Field of Invention
The present invention pertains to an automated windshield wiper system, and more particularly to a windshield wiper system which is automatically activated by moisture on the exterior surface of the windshield.
2. Prior Art
An effective windshield wiper system is essential for the operational safety of both vehicle and passengers. Modern windshield wiper systems provide a variety of speeds which permit the driver to adjust the rate of operation to specific weather conditions. For example, a typical switch or hand lever on the steering column of a motor vehicle has four positions. The off position disables the drive motor and discontinues operation of the wipers. High and low speed operation is provided for use in heavy to moderate rain or snow fall. In addition, many vehicles are equipped with an intermittent switch which permits adjustment of wiper operation. On various intermittent cycles the rate of operation may extend from a one to ten second time lapse between wiper reciprocations. This intermittent switch enables the driver to selectively adjust the wiper rate to a frequency of his own choosing, based on the driver's tolerance for moisture build up and wiper interference with his view.
In all cases with the aforementioned operating systems, the rate of reciprocation of the wipers is constant. Each reciprocation is followed by a specific time lapse which remains uniform until a different switch setting is applied by the driver.
It is rare, however, to find that a specific setting will meet the requirements of maintaining the windshield free of moisture, without frequent adjustment. For example, the rate of rain or snow fall can quickly change. Furthermore, passing vehicles may splash water from the road to cause an unexpected interruption to the driver's ability to see through the windshield. Substantial reaction time is often required to reach for the switch and make the appropriate speed adjustment to clear the window of moisture.
Because of the inconvenience of making adjustments to windshield wiper speed, several attempts have been made to provide a sensing system to automatically activate the wipers upon occurrence of a given amount of moisture on the windshield. U.S. Pat. No. 3,649,898 discloses a wiper control apparatus which includes two wires laid on the surface of the windshield. Wiper operation activates when a drop of water falls between the wires such that a short is created, providing an electrical path between the wires on the windshield. As the wipers pass over the water droplet, the wiper operation is disabled.
A similar design approach is disclosed in U.S. Pat. No. 4,317,073 by Blaszkowski. This reference teaches the use of exterior sensors to actuate low or high speed in accordance with the degree of rain or moisture precipitation. Sensors are primarily responsive to conductance between electrodes, or are sensitive to variations in temperature. German Pat. No. 2,057,062 also teaches the use of conductance for sensing moisture at the windshield surface. This reference discusses the use of adhesive foils separated at the windshield surface, the system being activated by measurement of resistance between the adhesive foil conductors.
The disadvantage of using conductance as a parameter for activating the windshield wipers arises from the requirement that the electrodes be placed on the exterior surface of the windshield. This location is essential in a conductance measurement because the moisture between the conductors is the variable factor, changing resistance and current flow between the conductors. Accordingly, the electrodes are directly exposed to corrosive influences of the elements, such as direct sunlight, moisture, smog, etc. In addition, mechanical separation of the electrode from the windshield surface may also arise because of exposure thereof to wind, wiper action, windshield cleaning materials, and comparable abrasive conduct applied at the windshield surface.
A second class of automatic wiper systems involve the measurement of capacitance. German Pat. Nos. 1,808,521 and 2,345,546 disclose circuits that involve measurement of capacitance between exposed plates. These references disclose the use of plates which are adhered to the interior windshield surface and which are coupled to a high frequency oscillator. Each of the latter references disclose the measurement of moisture content at the exterior windshield surface by high frequency differentiation. Here again, however, the positioning of plates at the exterior surface of the windshield creates risk of inadvertent removal or abrasion and insufficient sensitivity. Also, the corrosive effects of the environment can effect capacitance or capacitive readings. Even on the interior surface of the window, risk of manual abrasion or tearing, as well as change of permittivity by the accumulation of oily films from airconditioners and the like, can adversely effect system operation. Where conductors or plates are thin, their positioning on exterior surfaces creates even a greater likelihood of breakage and separation. If wider conductors are used, driver view is obstructed. This will be particularly true of conductors positioned on the interior surface of the windshield. In addition to the sensitivity and wear factors previously discussed, consideration must be given to cost of the activating system. Because this is a convenience item, excessive cost will defeat its application in most vehicles. What is needed, therefore, is an inexpensive detection system which is not exposed to abrasion, wear and tear; yet is capable of measuring moisture build up at the windshield surface.